1. Field of the Invention
The present invention relates to a method of manufacturing a semitransparent display device and a semitransparent display device, and particularly to improvements in a manufacturing method and a structure of a semitransparent display device, which has a transparent display region and a reflective display region, for increasing a display quality of the reflective display region.
2. Description of the Background Art
Display devices used in portable information terminals called PDAs (Personal Digital Assistants) and portable communication terminals such as cellular phones are required to have thin and light structures, to operate with low power consumption and to achieve high visibility in various environments. Liquid crystal displays, which can inherently achieve thin and light structures as well as low power consumption, have been widely used as the display devices of such portable terminals.
The liquid crystal display device differs from a CRT (Cathode-Ray Tube), a PDP (Plasma Display Panel) and others in that a liquid crystal panel itself, which provides image information, does not emit light. Therefore, the liquid crystal display devices can be roughly classified into two types, i.e., a transparent type allowing passing of light emitted from a light source called a backlight, and a reflective type using a reflector for reflecting incoming external light.
The reflective type can provide a high visibility in a bright place, but cannot avoid remarkably lowing of visibility in a dark place. Conversely, the transparent type cannot provide a high visibility in a bright place, and further requires a larger amount of energy consumption because the backlight consumes a power accounting for a large proportion of the whole power consumption of the liquid crystal display device.
In view of the above, a liquid crystal display device of a semitransparent type has been developed. In semitransparent type, a transparent display region allowing passing of light emitted from a backlight is formed in a portion of a display region, and a reflective display region reflecting incoming external light is also formed in the other portion of the display region. This type of liquid crystal display device having both the features of the transparent and reflective types can provide high visibility in various environments.
In general, a liquid crystal panel includes a TFT (Thin Film Transistor) array substrate, which is provided with a large number of thin film transistors formed in a matrix form on a glass substrate, a color filter, which has color patterns formed on a glass substrate and is arranged over the TFT array substrate, and liquid crystal filling a space between the TFT array substrate and the color filter. This liquid crystal display device performs image displaying by controlling orientation of the liquid crystal within the space.
The liquid crystal display devices of the above type are disclosed, e.g., in Japanese Patent Laying-Open Nos. 2000-29030, 2000-171794, 2000-180881, 2000-284272, 2001-221995 and 2001-350158.
In a process of manufacturing the TFT array substrate of the conventional semitransparent liquid crystal display device, thin film transistors, each of which is formed of a gate electrode, a gate insulating film, a semiconductor layer, a source electrode and a drain electrode, and an interlayer insulating film are formed on a glass substrate. For forming an organic flattening film on the glass substrate, a photosensitive organic material is applied over the glass substrate, and exposure processing is effected thereon.
The exposure as well as development and burning processing are effected on the photosensitive organic material thus applied to form the organic flattening film having through openings in a transparent display region and a contact region. By forming transparent electrodes on the transparent display region, the region can function as a display device of the transparent type. A reflective film may be formed on the organic flattening film and the contact region to provide a reflective display region, whereby this region can function as a display device of the reflective type.
In the processing of exposing the photosensitive organic film, UV (ultraviolet) light is emitted to the transparent display region and the contact region through a photomask to expose only the photosensitive organic film arranged over these regions to the emitted light so that an intended photosensitive organic film can be formed. However, the emitted UV light passes through the gate insulating film made of a silicon nitride film and the interlayer insulating film, further passes through the glass substrate, and reaches a stage carrying the glass substrate.
Therefore, the light reflected by the stage reenters the TFT array substrate through its backside, and exposes the photosensitive organic film. When the photosensitive organic film arranged over portions other than the transparent display region and contact region is exposed to the reflected light, which cannot be controlled by the mask pattern as described above, this results in a problem that the film thickness of the organic flattening film is locally reduced.
Particularly, the stage is provided with grooves, e.g., of about 1 mm in depth for vacuum suction of the TFT array substrate as well as detection portions for various sensors. If the stage has the grooves, light reflected outside the grooves and light reflected inside the grooves have different intensities, and thus reduce the thickness of the organic flattening film by different amounts, respectively. Therefore, the reflective display region, which is provided by forming the reflective film on the organic flattening film, exhibits variations or irregularities in visibility of display due to a difference in reflectance, which is caused by the grooves and others on the stage, so that the display quality lowers.